The circuitry of protein tyrosine phosphorylation underlies many fundamental cellular processes, such as signal transduction, cell division, and differentiation. The primary components of these circuits are protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). A broad outline of the complex interplay between these enzymes is understood, but the details of this relationship, particularly regarding the PTPs, are only beginning to emerge. The goal of this study is to examine the molecular mechanisms by which an abundant, prototypic intracellular PTP (PTP1B) regulates cell growth and metabolism, with special emphasis on its ability to regulate integrin, growth factor, and cytokine signaling. We have found that PTP1B contains at least two separate substrate-binding motifs, which govern interactions with its targets. This unexpected finding suggests that PTP1B regulates growth and metabolic signals by binding to distinct classes of proteins. These findings have wide-ranging implications for growth control and have completely altered our view of PTP1B's mechanism of action in the cell. Thus, we now propose a series of experiments to systematically explore the nature and scope of PTP1B's partners and their role in regulating cell proliferation. One PTP1B target, the tyrosine kinase Src, plays a central role in both RPTK and integrin signal transduction. We plan to explore the relationship between the substrate binding domains on PTP1B and the activity of Src. We will also examine the effects of PTP1B on cytokine signaling, which we believe involves a new substrate of PTP1B. These experiments will be pursued both in vitro and in vivo, by expressing various PTP1B binding-domain mutants in PTP1B-null cells and animals. The use of such genetically characterized systems provides an ideal, 'noise-free' background for these experiments, and should greatly facilitate the analysis of PTP1B-regulated signal transduction pathways. We anticipate that the insights gained from these studies of PTP1B will fill an important gap in our understanding of protein tyrosine phosphorylation and its role in cellular metabolism and growth control. Importantly, it also may point the way to the development of new therapeutic agents for diseases such as diabetes and cancer.